Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and devices used for monitoring a seismic array's geometry and adjusting trajectory of components thereof to achieve a target geometry and thereby an intended source signature.
Discussion of the Background
Seismic surveys are used to investigate underground formations by generating seismic waves and measuring reflected waves (i.e., their travel time, amplitude, direction, etc.). Seismic data is processed and converted into structural information about the underground formation.
Time-lapse seismic surveying is increasingly used to evaluate hydrocarbon-bearing underground reservoirs' evolution. In time-lapse seismic surveying, seismic data related to the same underground target volume is acquired at least twice over a given time (e.g., months or even years). The time-lapse seismic survey data is considered to be four dimensional (4D) because time between seismic surveys (or survey index) is an additional dimension to the typical three-dimensional seismic data acquired during each survey. The earliest of the seismic surveys is known as the “base survey” (during which base data is acquired), and subsequent seismic surveys are known as monitor surveys (during which monitor data sets are acquired).
Differences observed in seismic data acquired during successive seismic surveys may be due to changes inside the surveyed target volume and/or data acquisition variations. Since changes inside the surveyed target volume are of interest, efforts to minimize data acquisition variations are ongoing. Various techniques have been developed to reproduce source and receiver locations of the base (or earlier) seismic survey during monitor (later) survey(s). For example, U.S. Patent Application Publication No. 2007/0247971 (the entire content of which is incorporated herein by reference) describes a method for monitoring and steering a source array during a monitor survey to follow the path of the source array during a previous (e.g., “base”) seismic survey. However, firing the source array at the same shot locations is not enough because the source array signatures may be quite different, as explained below.
FIG. 1 illustrates a marine seismic data acquisition system including a vessel 10 towing a source array 20 and streamers 30 (not all are labeled) along a sail-line S. Source array 20 includes plural sub-arrays 21-26, which are described in more detail below. Streamers 30 are connected to vessel 10 via lead-in cables 32A and 32B, which are pulled laterally by deflectors 34A and 34B.
A single sub-array 200 (that may have a 25-50 m length and include 6-8 individual seismic sources) is illustrated in FIG. 2. Sub-array 200 includes a float 210 and individual seismic sources 220a-g (that may have different characteristics). Plural floats may be used, for example, in seismic data acquisition in shallow water. Each of the individual seismic sources is attached to one of source base-plates 225a-g via pairs of cables 222a-g (only 222a and 222g labeled), and source base plates 225a-g are suspended under float 210 via cables 227a-g. Plural individual seismic sources (i.e., a cluster of individual seismic sources) may be attached to any one of the base-plates. The source base plates are connected to each other via links 230a-g (only 230a, 230b and 222g labeled) and to the towing vessel via an umbilical 240 to receive and distribute electric power and compressed air to the individual seismic sources. The individual sources may be attached to a single stiff base instead of being attached plural base plates. Note that the seismic source may also have no float as described, for example, in U.S. Pat. No. 7,457,193 (the entire content of which is incorporated herein by reference).
The individual seismic sources are fired such that, at a great enough distance from the source array, primaries (i.e., first oscillation) from each individual source interfere constructively, while secondaries (i.e., subsequent oscillation) interfere destructively, with pressure variations caused by the individual sources merging in a seismic wave. Beyond a certain distance (e.g., more than 100 m, which is far greater than the source's dimensions), this seismic wave has a stable frequency-versus-amplitude shape, its amplitude then decreasing inversely proportional to the distance from the source (as is the case for a point-like source). The frequency-versus-amplitude shape that no longer changes significantly with distance is known as the source signature. The source signature is a basic characteristic of seismic data acquisition because the seismic waves detected in response to this wave are a convolution of the source signature with the target volume response. The source signature also varies with the azimuth angle (i.e., the angle from a tow axis to a line between the shot location and a receiver location).
The source signature depends on characteristics of the individual seismic sources (e.g., air gun or air gun cluster type, volume, pressure etc.), their geometrical arrangement (e.g., inline distances between the individual seismic sources in a sub-array, distance between the sub-arrays, depths of the individual seismic sources, etc.), the firing sequence of the individual seismic sources and environment conditions (e.g., waves, currents, etc.). Some of these parameters, such as the characteristics of individual seismic sources or the firing sequence, can be reliably reproduced in successive surveys, thus ensuring that these parameters do not alter the source signature from one survey to another survey. However, some other parameters, such as environment conditions, cannot be controlled, but the effect of these parameters on the source signature may be alleviated during data acquisition or taken into account during seismic data processing. For example, the depth of the source array may be adjusted depending on wave height (i.e., an environmental condition that cannot be controlled) to achieve the same source signature.
One parameter affecting the source signature that conventionally has not been a concern is the source array's geometry. However, in fact, this geometrical arrangement may vary from shot to shot and has a significant impact on the source signature. FIGS. 3A-3C schematically illustrate source array's geometry changes that may occur during seismic data acquisition and alter the source array's signature. In FIG. 3A (which is a bird-eye view), a distance d1 between sub-arrays 310 and 320 at a first location, O, along sail-line 330 is smaller than a distance d2 between sub-arrays 310 and 320 at a second location, O′. In FIG. 3B (which is also a bird-eye view), at a first location, O, sub-array 310 is substantially parallel to sail-line 330, and then, at a second location, O′, sub-array 310 makes a non-zero angle β with sail-line 330. In FIG. 3C (which is a vertical plane view as suggested by the direction of gravitational acceleration g), sub-arrays 310 and 320 are at a same depth h1, at a first location, O, but then, sub-array 310 is at a depth h1>h2, at a second location, O′. In yet another example, the sub-arrays inline position (i.e., position along sail-line) is a parameter that affects the source signature.
In order to produce an intended source signature (e.g., reproduce the source signature during a previous survey when acquiring 4D seismic data) is would be desirable to control the source array's geometry while acquiring seismic data.